rustdoc: add tooltips to sidebar

[liigo]

This PR add tooltip to most every links in sidebar, showing its primary description.

Online preview:
http://liigo.com/tmp/rust/std/index.html
http://liigo.com/tmp/rust/std/str/trait.StrSlice.html

[steveklabnik]

I like this, though it shows the raw Markdown, which is awkward.

[liigo]

@steveklabnik Are you referring the

`code` in docs

and # header ?

For the first one, I think it's still readable, better than "code in docs". What style are you like to change to? For the second one, there are only several # header style in tooltips, and there are come from search-index.js, which is out of this PR, but i'd like seeing it be changed later.

[steveklabnik]

I'm referring to the fact that the tooltips contain the literal text foo bar [some link](https://whatever.com/whatever), as well as code in docs and #header stuff.

[liigo]

@steveklabnik Oh, yes, there are markdown links [title](url) and [title][ref]. Perhaps we can fix this by removing these links in brief docs?

[liigo]

cc @pcwalton @alexcrichton @brson
Currently, I don't know how to do with these Markdown codes, which come from doc-comments. Parse it or just remove it (off brief docs)?

[alexcrichton]

Closing due to inactivity, but feel free to reopen with a rebase!